Dual-frequency helix antenna

ABSTRACT

An antenna (400; 600) for transmitting and receiving radio-frequency signals comprises a cylindrical coil conductor (601) having a turn A and a turn B and between them other turns. The pitch (x1) of turn A is unequal to the pitch (x2) of said turn B, and the pitches of the other turns between turns A and B are in the order of magnitude between the pitches of turns A and B.

FIELD OF INVENTION

The invention relates in general to antenna structures in radioapparatus. In particular the invention relates to an antenna structurewhich has two resonating frequencies different from each other. Thispatent application uses a mobile phone as an example of a radioapparatus.

DESCRIPTION OF RELATED ART

In different parts of the world there are cellular radio systems in usethat differ from each other significantly in their operating frequencyranges. As regards digital cellular radio systems, the operatingfrequencies of the Global System for Mobile Telecommunications (GSM) arein the 890-960 MHz range, the operating frequencies of the JapaneseDigital Cellular (JDC) system are in the 800 MHz and 1500 MHz bands, theoperating frequencies of the Personal Communication Network (PCN) are inthe 1710-1880 MHz range, and those of the Personal Communication System(PCS) in the 1850-1990 MHz range. The operating frequencies of theAmerican AMPS mobile phone system are between 824 MHz and 894 MHz andthose of the Digital European Cordless Telephone (DECT) system in the1880-1900 MHz range.

Since the resonating frequency of a prior-art radio-frequency antennadepends in a known manner on the length of the antenna, through thewavelength, a particular antenna can be used only in a mobile phonedesigned for a single-frequency cellular radio system. In some cases,however, it is desirable that one and the same phone could be used insome other frequency range, too. In addition to other suitable RF parts,a working antenna arrangement is then needed.

U.S. Pat. No. 4,442,438 discloses an antenna structure resonating at twofrequencies, comprising, as shown in FIG. 1, two helices 101, 102 andone whip element 103. The helices 101 and 102 are positioned one afterthe other and their adjacent ends 104 and 105 constitute the feed pointof the combined structure. The whip element 103 is partly inside theupper helix 101 and its feed point 106 is at its lower end. An RF signalis brought to the feed point 106 via a coaxial conductor 107 coincidingwith the symmetry axis of the structure and traveling through the lowerhelix 102. The feed point 106 of the whip element is coupled to thelower end 104 of the upper helix, and the lower helix is coupled at itsupper end 105 to the conductive and grounded shroud of the coaxialconductor 107. The structure's first resonating frequency is theresonating frequency of the combined structure of helices 101 and 102;827 MHz in the illustrative embodiment. The second resonating frequencyof the structure is the common resonating frequency of the upper helix101 and the whip element 103; 850 MHz in the illustrative embodiment.Thus, helix 101 and whip element 103 are such that they havesubstantially the same resonating frequency.

The structure disclosed by the US patent is relatively complex. From themanufacturing standpoint, the most difficult part in the structure isthe feed point arrangement at the middle of the antenna, where the lowerend 106 of the whip element and the lower end 104 of the upper helixhave to be galvanically coupled, and the lower helix has to be coupledat its upper end 105 to the shroud of the coaxial conductor feeding thewhip element. According to the material presented in the patent thedifference between the two resonating frequencies achieved by thestructure is small because the dimensions of the upper helix 101 and thewhip element 103 have to be such that they have substantially the samecommon resonating frequency, so the structure cannot be applied to aphone operating at the GSM and PCN frequencies, for example. Indeed, inthe description of the patent it is stated that an object of theinvention is to broaden the resonating frequency area of the mobilephone antenna such that it would better cover the whole frequency rangein one cellular radio system.

FI patent application 963275 (LK-Products) discloses a dual-frequencyantenna structure according to FIG. 2 in which there is at a certainpoint between the ends of a helix antenna 201 wound into a cylindricalcoil a coupling part 202 for coupling to a second antenna element 203.The cylindrical coil conductor 201, which is the first antenna elementin the antenna, comprises in the direction of its longitudinal axis alower part 204 and an upper part 205, and the second antenna element 203is connected to the cylindrical coil conductor through a fixed couplingat the coupling point 202 between the lower and upper parts. The tworadiating antenna elements of the structure have a common lower part upto the branching point consisting of the coupling part, from which pointon the electrical lengths of the antenna elements are different. Thefirst resonating frequency of the combined antenna structure isdetermined by the total electrical length of the common lower part ofthe antenna elements and the upper part of the first antenna element.The second resonating frequency is determined by the total electricallength of the common lower part of the antenna elements and the upperpart of the second antenna element. In addition, the resonatingfrequencies are affected by the mutual coupling of the antenna elementsand the fact that the antenna elements are electrically conductivebodies in the near fields of one another so that they put a load on eachother. The antenna structure according to FIG. 2 is relatively difficultto precisely dimension to the desired frequencies since the couplingpoint between the antenna elements requires quite accurate positioning.In addition, the electrical coupling in the coupling point easilybecomes unreliable.

FI patent application 970297 (LK-Products) discloses an antennaaccording to the principle illustrated in FIG. 3 wherein an antennaelement 301 has a first end and a second end and a tapping point 302which is located at a certain point between the ends of the antennaelement. The tapping point divides the antenna element asymmetricallysuch that the electrical length from the tapping point to the upper endis considerably greater than the electrical length from the tappingpoint to the lower end. The feed conductor 303 of the antenna, whichconnects the antenna element electrically to a radio apparatus, iscoupled to the antenna element at the tapping point. A substantialportion of the feed conductor also serves as a radiating element becausethe feed conductor is electrically unshielded, i.e. it has no shroudmade of a conductive material around it. The total electrical length ofthe antenna structure at a first operating frequency is the sum of theelectrical lengths of the feed conductor 303 and the portion extendingfrom the tapping point 302 to a first end of the antenna element 301.Correspondingly, the total electrical length of the antenna structure ata second operating frequency is the sum of the electrical lengths of thefeed conductor 303 and the portion extending from the tapping point 302to a second end of the antenna element 301. The antenna element 301 maybe a helix, a straight conductor or a combination of those. Thedisadvantage of this antenna structure is the difficulty inmanufacturing the antenna structure such that the tapping point 302 willbe sturdy.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an antenna structurewhich can be applied in two operating frequency ranges and which issimple to manufacture and reliable in its operation. Another object ofthe invention is to provide an antenna structure which can be easilydimensioned to two different operating frequencies. A further object ofthe invention is that the antenna structure according to the inventionis applicable to large-scale series production.

The objects of the invention are achieved by using as an antenna elementa helix the pitch of which decreases when moving away from the feedpoint.

The antenna according to the invention comprises a cylindrical coilconductor having a turn A and turn B and other turns between them. Theantenna is characterized in that the pitch of turn A does not equal thepitch of turn B and the pitches of the other turns between turn A andturn B are arranged according to the magnitude between the pitch of turnA and the pitch of turn B.

It is known that a conductive body may have multiple resonatingfrequencies the lowest one of which is the so-called fundamentalfrequency, the rest being harmonic frequencies. The invention is basedon the observation that the resonating frequency of a cylindrical coilconductor, or helix, is changed when the dimensional parameters of thehelix are changed in the various parts of the structure. The electricallength of the helix conductor determines the fundamental frequency. Inconnection with helices, the distance between the ends of a turn in thedirection of the longitudinal axis of the helix is called a pitch. Whenthe feed point is at one end of a helix and the pitch either decreasesor increases towards the other end, the mutual interaction of the turnschanges the resonating frequencies. When the number of turns, pitch ofthe helix at various points and other parameters are suitably selected,the resonating frequencies will be at such positions on the frequencyaxis that the structure can be used in two cellular radio systemfrequency ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to thepreferred embodiments presented by way of example and to theaccompanying drawing wherein

FIG. 1 shows a known antenna structure,

FIG. 2 shows a second known antenna structure,

FIG. 3 shows a third known antenna structure,

FIG. 4 shows the principle of the invention,

FIG. 5 shows measured properties of the structure according to FIG. 4,and

FIG. 6 shows the antenna according to the invention with a protectivehousing.

Above in conjunction with the description of the prior art reference wasmade to FIGS. 1 to 3, so below in the description of the invention andits preferred embodiments reference will be made mainly to FIGS. 4 to 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 shows a longitudinal section of a helix antenna 400 having seventurns. Viewing from the feed point 401 the pitch x1 of the first turn isgreater than the pitch x2 of the last turn. The pitches of the otherturns decrease evenly from the first turn toward the last turn. In FIG.4 the helix antenna is shown in the upright position but the inventiondoes not limit the use or manufacture of the helix antenna according tothe invention in any particular position. A feed point 401 and the leg402 of the helix can be realised in such a manner that the helixconductor is bent into the shape of the black line shown in the Figure.In an alternative implementation the helix is connected at its bottomend, with respect to the position shown, to a coupling part having acylindrical hollow into which the lowest turns of the helix areinserted. To that end, the bottom end of the helix may have a supportthread (not shown) more densely wound than the rest of the helix, saidsupport thread, when connected to the coupling part, will not serve asradiating element as the electrically conductive coupling part shortcircuits the turns of the support thread. Other known methods forcreating a feed point 401 and for connecting the helix antenna to aradio apparatus can be used, too.

FIG. 5 illustrates a measurement of the so-called s11 coefficient, orreflection coefficient, with the horizontal axis representing thefrequency range of 700 MHz to 2100 MHz and the vertical axisrepresenting the value of the reflection coefficient in units ofdecibel. The measurement concerns an antenna according to FIG. 4. Thetriangular symbol on the vertical axis represents 0 dB, one step on thevertical axis equals 5 dB and one step on the horizontal axis equals 140MHz. The reflection coefficient tells how much of the radio-frequencypower fed to the antenna via the feed point is reflected back. A lowvalue of the reflection coefficient at a certain frequency means theantenna is suitable for that frequency. FIG. 5 shows that the antennahas two resonating frequency ranges wherein the value of the reflectioncoefficient is clearly smaller than −10 dB. The first resonatingfrequency range (s11<−10 dB) is about 880 MHz to 960 MHz, and the secondresonating frequency range (s11<−10 dB) is about 1730 MHz to 1800 MHz.

Instead of becoming denser the turns of the helix may also becomethinner, i.e. the pitch may increase from the feed point on. Theresonating frequency ranges of the antenna according to the inventiondepend among other things on the thickness of the helix conductor, pitchof the turns and on the diameter of the helix. The table below showssome measurement results for helices H1, H2, H3, H5, H6, H7, H8, H9, andH10 in which the height of the helix from the beginning of the firstturn to the end of the last turn is 22 mm, the length of the leg (402 inFIG. 4) of the helix is 10 mm, and the thickness of the helix conductoris 0.9 mm, as well as for a helix H11 in which the height of the helixis 16 mm, thickness of the helix conductor is 0.9 mm, height of the legis 6 mm and the diameter of the leg is 3 nun, as well as for a helix H12in which the height of the helix is 16 mm, thickness of the helixconductor is 0.8 mm, height of the leg is 6 mm and the diameter of theleg is 3 mm. The lower and upper diameter values shown in the table areinner diameters and the frequencies f1 and f3 are the resonatingfrequencies in the frequency ranges for which the helix is suitable.

H1 H2 H3 H5 (decr. pitch) Lower diameter/mm 7.1 × 7.1 2 × 2 3 × 3 7.1Upper diameter/mm 7.1 × 7.1 8.2 × 8.2 14 × 14 7.1 Pitch/mm 4 2.5 5 5 +4.5 + 4 + 3.5 + 2.3 + 2 Outer volume/mm³ 1110 620 1530 1110 Freq./Realpart of imp. f/MHz Re/Ω f/MHz Re/Ω f/MHz Re/Ω f/MHz Re/Ω Resonance f1935.1 43 902.9 54 893.9 56 898.5 55 Resonance f3 2213 12 2011 21 2046 191812 23 Ratio f3/f1 2.37 0.28 2.23 0.39 2.29 0.34 2.02 0.42 H6(decr./pitch) H7 (incr./pitch) H8 (incr./pitch) H9 Lower diameter/mm 7.17.1 7.1 7.1 × 7.1 Upper diameter/mm 7.1 7.1 7.1 2 × 2 Pitch/mm 6.5 + 5 +3.5 3 + 3.5 + 4 2 + 3 + 4 + 5 2.3 + 2.7 + 2 + 1.8 + 4.4 + 4.6 + 6 + 7Outer volume/mm³ 1110 1110 1110 510 Freq./Real part of imp. f/MHz Re/Ωf/MHz Re/Ω f/MHz Re/Ω f/MHz Re/Ω Resonance f1 906.0 55 905.9 47 889.6 48911.4 43 Resonance f3 1771 28 2255 12 2379 10 2371 10 Ratio f3/f1 1.950.51 2.49 0.26 2.67 0.21 2.60 0.23 H10 H11* H12** Lower diameter/mm 7/1× 7/1 5/1 × 5/1 6.2 × 6.2 Upper diameter/mm 5 × 5 5.1 × 5.1 5.4 × 5.4Pitch/mm 3.1 1.7 3.5 + 3.0 + 2.4 + 2+ 1.5 + 1.2 + 1, 1 + 1 Outervolume/mm³ 830 450 550 Freq./Real part of imp. f/MHz Re/Ω f/MHz Re/Ωf/MHz Re/Ω Resonance f1 902.9 48 911.1 20 901 21 Resonance f3 2203 102081 12 1801 11 Ratio f3/f1 2.43 0.21 2.28 0.6 2.0 0.52 * and **:dimensions different from the other helices, see above

In the table, the pitch of the helices H1, H2, H3, H9, H10 and H11 isthe same in all turns, i.e. they are not in accordance with theinvention. In helices H2, H3, H9, H10 and H12 the diameters of the turnschange between the feed point and the second end of the helix: the lowerdiameter refers to the diameter nearest to the feed point. The values ofthe ratio f3/f1 printed in boldface emphasize helices H5, H6 and H12which from the resonating frequency standpoint are especially suitableas antennas for a GSM/PCN dual-mode phone.

FIG. 6 shows in the form of a longitudinal section an antenna 600according to the invention comprising a helix conductor 601, couplingpart 602 made of metal or another electrically conductive material, anda protective housing 603. The outer surface of the coupling part 602 hasthreads 604 whereby the antenna 600 can be mechanically and electricallycoupled to a radio apparatus (not shown). The lower part of the helixconductor has a dense support thread 605 whereby the helix conductor 601is attached to a cylindrical hollow in the coupling part 602. Thesupport thread does not belong to the radiating portion of the antenna.The protective housing 603 is made of a dielectric material, preferablyinjection-molded plastic, and it can be attached to the coupling partwith glue or by means of fusion welding. The protective housing 603 mayinclude components (not shown) supporting the helix conductor 601, suchas a cylindrical pin pushed inside the helix from the top.

The present invention is not limited to the exemplary embodimentsdescribed here, nor to any particular application but can be used inantennas in different applications and at different frequencies,advantageously radio frequencies such as UHF and VHF. The structure isadvantageously used in antennas of mobile phones. The structure may bemodified within the scope of the invention defined by the claims setforth below. The pitches of the first and last turns of the helix mayeven be almost identical if there is a second turn between them having apitch unequal to that of the first turn, if then there are other turnsbetween the first and said second turn where the pitch changes in aregular manner.

What is claimed is:
 1. An antenna for transmitting and receivingradio-frequency signals, comprising: a cylindrical coil conductor havinga first and second turn and one or more additional turns between saidfirst and second turn; said first and second turn each having a pitchunequal to the pitch of the other of said first and second turn; andsaid one or more additional turns each having a pitch unequal to thepitch of the other of said additional turns; and said respective pitchesof said one or more additional turns having values between therespective values of the pitches of said first and second turns; whereinsaid turns and pitches are arranged such that the fundamental resonantfrequency of the antenna is in the operational frequency range of afirst cellular radio system and a harmonic resonant frequency of theantenna is in the operational frequency range of a second cellular radiosystem.
 2. The antenna of claim 1, wherein said antenna includes a firstand second end; said first end located at said first turn and saidsecond end located at said second turn; said first end comprising a feedpoint of said antenna.
 3. The antenna of claim 2, wherein the value ofsaid pitch of said second turn is less than the value of the pitch ofsaid first turn, whereby the value of the pitch of succeeding respectiveturns decreases as the distance between said respective turns and saidfeed point increases.
 4. The antenna of claim 1, wherein said antennacomprises a first and second resonating frequency and operates in afirst and second operating frequency band of a cellular radio system,said first resonating frequency being substantially similar to saidfirst operating frequency band; and said second resonating frequencybeing substantially similar to said second operating frequency band. 5.The antenna of claim 1, further including a coupling part having acylindrical hollow for receiving said first end of said cylindrical coilconductor of said antenna.
 6. The antenna of claim 5, wherein said firstend of said cylindrical coil conductor of said antenna includes asupport thread for coupling to said cylindrical hollow of said couplingpart.